Sealing device for an instrumentation column

ABSTRACT

A sealing device comprising a supporting assembly (9) containing sealing members (16) supporting the instrument column (5), and an assembly (17, 18) for lifting the instrument column (5) in sealing engagement with the supporting members (16). The sealing members (16) supporting the instrument column (5) include a clamping ring (28) and at least one O-ring seal (29, 30). Axial movement of the instrument column (5) enables clamping of the inner seal (30) via the clamping ring (28). When axial movement of the instrument column (5) is restricted, the seals may be clamped by means of a suitably sized clamping ring (28).

FIELD OF THE INVENTION

The invention relates to a sealing device for an instrumentation column,and in particular for a thermocouple column penetrating the head of apressurized-water nuclear reactor vessel.

BACKGROUND OF THE INVENTION

In pressurized-water nuclear reactors, the vessel containing the core ofthe reactor includes a head of substantially hemispherical shape havingopenings in which there are fixed adapters allowing the passage of thecontrol rods of the reactor and instrumentation columns such asthermocouple columns of cylindrical shape. In each of the thermocouplecolumns there is arranged a set of thermocouples making it possible tomeasure the temperature of the coolant fluid at the outlet of a set offuel assemblies of the core of the reactor.

Each one of the adapters includes a part projecting under the headproviding guidance for the thermocouple column and a part projectingabove the head including means for connecting a tubular bearing assemblyof the thermocouple column, which assembly can be fixed in the extensionof the adapter.

Inside the bearing assembly fixed to the adapter are arranged sealedbearing means intended to interact with a surface of the thermocouplecolumn in order to provide the sealed passage of the thermocouple columnthrough the bore in the bearing assembly extending the adapter in theaxial direction.

The thermocouple column which passes through the bore in the bearingassembly and the bore in the adapter which are situated in the axialextension of one another includes an end which projects relative to thebearing assembly which is in engagement with a lifting device whichworks by pulling, allowing the thermocouple column to be moved in theaxial direction. The bearing surface of the thermocouple column and thesealed bearing means of the bearing assembly can thus be brought intocontact, with a certain pressure.

The sealed bearing means of the thermocouple column inside the bearingassembly generally consist of a bearing surface of frustoconical shapemachined into the internal bore of the bearing assembly and the shape ofwhich complements the shape of the bearing surface of the thermocouplecolumn, and of a gasket which is compressed between the bearing surfacesof the thermocouple column and of the bearing assembly when clamping isperformed by pulling on the upper end of the thermocouple column.

A drawback of such a sealing device is that it requires a certain amountof axial movement of the thermocouple column between its loweredunclamped position and its raised, leaktight clamped position, thisdistance of axial movement being defined rather vaguely by the positionof the bearing surface inside the bearing assembly relative to theposition of the bearing surface of the thermocouple column in theunclamped position.

After the nuclear reactor has been in use for a certain length of time,or after a certain number of operations of opening the vessel byremoving the head, some thermocouple columns may become slightlydeformed, so that they are no longer perfectly straight.

In this case, it may be difficult or even impossible to move thethermocouple column axially inside the adapter and the bearing assemblyover sufficient distance to provide clamping of the sealing gasket.

It then becomes necessary to exchange the entire thermocouple column,which leads to substantial additional costs in the operation of thenuclear reactor.

To date, no device was known for sealing an instrumentation column suchas a thermocouple column penetrating the head of the vessel of a nuclearreactor which allows sealed clamping of the column even in cases wherethe latter has suffered deformation which prevents sufficient axialmovement inside the adapter to allow the sealing gaskets to becompressed.

FR-A-2,642,217 describes a sealing device for an instrumentation columnpenetrating the vessel head of a nuclear reactor in which the bearingassembly consists of two successive lengths which are assembled in sucha way that the instrumentation column can be extracted quickly andeasily even in the event of seizure of the parts for clamping andtightening the bearing assembly onto the end of the adapter.

However, in this improved device, no means have been provided to make itpossible, if need be, to reduce the axial movement required for ensuringsealed clamping of the instrumentation column.

SUMMARY OF THE INVENTION

The object of the invention is to provide a sealing device for aninstrumentation column, and, in particular, for a thermocouple columnpenetrating the head of a pressurized-water nuclear reactor vessel,inside an adapter of tubular shape fixed into a penetration opening ofthe head and projecting outside the vessel head, including a tubularbearing assembly fixed to the end of the adapter outside the head and inits axial extension as well as a means for pulling on an end of theinstrumentation column which projects outside the bearing assembly,allowing the instrumentation column to be moved in the axial directionof the adapter, in order to bring a bearing surface of theinstrumentation column into contact with sealed bearing means arrangedinside the bearing assembly, it being possible for this device to sealthe instrumentation column even if this instrumentation column hasdeformation which limits the axial movement of the instrumentationcolumn inside the adapter and the bearing assembly.

To this end, the sealed bearing means of the instrumentation columninclude:

a clamping ring having a contact surface, the shape of which complementsthe shape of the bearing surface, of the thermocouple column mounted sothat it can move in the axial direction, inside the bore of the bearingassembly, so that its contact surface faces the bearing surface of theinstrumentation column inside the bearing assembly, and

at least one annular sealing gasket interposed between a face of theclamping ring opposite its contact surface and an internal surface ofthe bearing assembly, so as to provide a seal between the externalsurface of the instrumentation column and the bore of the bearingassembly, through interaction of the clamping ring, of the sealinggasket and of the internal surface of the bearing assembly under theeffect of an axially-directed force exerted by the instrumentationcolumn coming into contact with the clamping ring as it is moved underthe effect of a pulling force exerted on its end.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to make the invention easy to understand, several embodimentsof a sealing device according to the invention will now be described byway of example with reference to the appended drawing figures.

FIG. 1 is a view in section on a vertical plane of a penetration throughthe head of a vessel of a nuclear reactor, allowing the passage of athermocouple column and equipped with a sealing device according to theinvention.

FIG. 2 is a view in side elevation of the upper part of the penetrationshowing the sealing device according to the invention.

FIG. 3 is a view in exploded perspective of the elements forming thesealing device.

FIGS. 4A and 4B are views in section on an axial plane of the sealingdevice.

FIG. 4C is an enlarged view of detail C of FIG. 4A.

FIG. 5 is a view from above in the direction of arrow 5 of FIG. 2.

FIG. 6 is a view in section along line 6--6 of FIG. 2.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows part of the vessel head 1 of a pressurized-water nuclearreactor through which there passes an opening 2 in which there is fixed,in a sealed manner, by welding, an adapter 3 including a part projectingbelow the vessel head and providing guidance for a thermocouple column 5and a part projecting above the vessel head 1 constituting a flared part4, on the outer surface of which a screw thread 4a is machined.

Fixed in the axial extension of the adapter 3 on the upper part 4 of theadapter 3 is an assembly 6 for fixing and sealing the thermocouplecolumn 5, by means of a lower part 7 or extension including a tappedbore which is engaged over the threaded part 4a of the flared part 4 ofthe adapter 3. The assembly for fixing and sealing the thermocouplecolumn 5 is thus assembled to the end of the adapter 3. The flared part4 of the adapter 3 and the lower part 7 of the fixing and sealingassembly 6 include circular joints 4', 7' which come into coincidencewhen the extension 7 is fully screwed onto the flared part 4. The joints4' and 7' are joined together by welding, so as to seal the screwedconnection between the components 4 and 7.

The head and the penetration adapters 3 are made at the factory andtransported to the site where the nuclear reactor is being installed.

The fixing and sealing devices 6 for the thermocouple columns 5 areattached and fixed to the upper ends of the adapters on the reactorsite.

The lower part of the fixing and sealing assembly 6 is fixed to the endof the adapter, in such a way as to be able to be removed if need be bymelting the junction region of the joints 4' and 7'. Such removal iscarried out only for exceptional interventions or repairs to thethermocouple column penetration piece.

The fixing and sealing assembly 6 includes an upper part 8 which isassembled in sealed manner with the extension 7 with the interpositionof a metal gasket 10 of special shape, the parts 7 and 8 of the fixingand sealing assembly being assembled using a clamping flange 12 made intwo parts which can be joined together and clamped using screwsintroduced into openings 13 passing through the opposing lugs situatedat the end of the two sector-shaped parts.

The clamping flange at its internal part has frustoconical bearingsurfaces which come into clamping contact with correspondingfrustoconical bearing surfaces machined on flared end parts 7a and 8a ofthe extension 7 and of the upper part 8, respectively, of the fixing andsealing assembly 6.

The upper part 8 of the fixing assembly 6 could be fixed to theextension 7 by welding or made as a single piece with the extension 7,or even consist of the upper part of the adapter 3.

The adapter 3 and the fixing and sealing assembly 6 are made in atubular shape and arranged in the axial extension of one another, so asto form a passage for the thermocouple column 5 consisting of a tube forsupporting and holding a set of thermocouples 15.

The upper part 8 of the fixing assembly 6 for the thermocouple columnconsists of a tube-shaped length of the bearing assembly 9 of thesealing device according to the invention. The bearing assembly 9 of thethermocouple column 5 additionally includes a cover 11 of annular shapeattached to the tubular length and fixed in a way which will bedescribed later. The tubular length 8 and the cover 11 of the bearingassembly 9 have bores which are arranged in the axial extension of oneanother and in the axial extension of the bore of the adapter 3.

The thermocouple column 5 has an upper part, the diameter of which issmaller than the diameter of its standing part, separated from thisstanding part by a shoulder of substantially frustoconical shapeconstituting the bearing surface 14 of the thermocouple column 5 insidethe bearing assembly 9.

Sealed bearing means 16 engaged inside the bore of the tubular length 8of the bearing assembly 9 come into contact with the bearing surface 14of the thermocouple column 5 in its clamped position.

The thermocouple column 5 is moved between its unclamped position andits clamped position by a compression plate 17 penetrated by tappedopenings and fitted with compression screws 18 engaged in the tappedopenings and coming to rest with their ends on the heads of screws 21for attaching the cover 11.

The plate 17 has a central opening allowing it to be engaged over theend part of the instrumentation column 5.

A pulling ring 19, the outside diameter of which is larger than thediameter of the central opening in the plate 17 and which consists oftwo half-rings engaged in a groove formed at the peripheral part of thethermocouple column in its upper part, allows the pulling force exertedby the plate 17 to be transmitted to the thermocouple column 5.

Reference will now be made to FIGS. 2, 3, 4A, 4B, 4C, 5 and 6 as awhole, for the description of the sealing device according to theinvention, which includes the bearing assembly 9, the sealed bearingmeans 16 of the thermocouple column 5 and the compression plate 17.

The sealing device has been represented in FIG. 2 in lateral elevationand in its position for clamping the thermocouple column.

In FIG. 3, the various elements of which the sealing device is formedhave been represented before assembly, in an exploded perspective view.

FIGS. 4A and 4B represent the sealing device in a view in section on anaxial plane. In FIG. 4A, the device has been represented with thethermocouple column clamped in a sealed manner against the bearingassembly.

In FIG. 4B, the device has been represented with the thermocouple columnin the unclamped position.

The upper part of the fixing assembly 6 consists of the bearing assembly9 and the compression plate 17 clamping the thermocouple column 5.

The bearing assembly 9 includes the tubular length 8 and the cover 11attached and fixed to the upper end of the tubular length 8.

As indicated earlier, the tubular length 8 may consist of a part whichis attached and fixed in the axial extension of an adapter forpenetrating the head of the vessel, either by means of a removablecollar, or by means of a weld.

The tubular length 8 on its lateral surface includes three recesses 20a,20b, 20c arranged at 120° from one another about the axis of the tubularlength 8, as is visible particularly in FIGS. 2, 3 and 6.

The recesses 20a, 20b, 20c emerge on the lateral surface of the tubularlength 8 and are each delimited by three flat surfaces, a lower flatsurface, an upper flat surface, and an internal flat surface.

As can be seen in FIG. 6, the three internal flat surfaces of the threerecesses 20a, 20b, 20c constitute three sides of the remaining sectionof the tubular length 8 at the recesses, which is substantially in theshape of an equilateral triangle.

The annular cover 11 constituting the upper part of the bearing assembly9 which has the shape of a disc is fixed to the tubular length 8 bymeans of three screws 21 placed at 120° from one another about the axisof the bearing assembly 9 which are engaged in openings 27 penetratingthe cover 11 and in openings placed in coincidence machined in thetubular length 8, so that each emerges in a recess 20.

Placed inside each of the recesses 20a, 20b and 20c is a correspondingfixing piece 22a, 22b, 22c including a tapped opening into which thethreaded end of the corresponding screw 21 is screwed.

In addition, each of the fixing pieces 22 on each side of its tappedopening has two holes for the passage of fixing shanks 31, having adirection parallel to the axis of the tubular length 8. The fixingshanks 31 include a threaded part which is screwed into a tapped hole ofthe tubular length 8 and a head allowing screwing which, when the shank31 is put in position, is housed in an opening emerging at the upperpart of the tubular length 8.

The openings penetrating the fixing pieces 22 have a diameter largerthan the diameter of the shanks 31, so that the fixing pieces 22 arerendered captive relative to the tubular length 8 by virtue of theshanks 31, but remain able to move in the axial direction inside therecesses 20 in order to clamp the cover 11 onto the upper end part ofthe tubular length 8.

Interposed between the cover 11 and the upper part of the tubular length8 is a torus-shaped gasket 29 housed inside a groove machined in theupper part of the tubular length 8.

The gasket 29 is a metal gasket of the Helicoflex type. Such aHelicoflex gasket is a flexible gasket including a torus-shaped jacketproduced by the non-closed rolling-up of a metal sheet inside whichthere is arranged a helical spring which causes the cross-section of thetorus to expand.

Upon clamping, the torus-shaped jacket and the helical spring arecompressed and provide a seal by elasticity.

The cover 11 is clamped by the screws 31, the head of which comes tobear inside openings 27 of cylindro-frustoconical shape passing throughthe cover 11.

The screws 21 have a countersunk head of frustoconical shape which, uponclamping, become fully housed inside an opening 27.

The threaded part of the screw is screwed into the tapped hole of thefixing piece 22.

Each of the fixing pieces 22 which have a flat internal face and acylindrical external face includes, in its cylindrical external surface,a groove 22' where the fixing piece and the screw can be cut throughusing a circular saw or a grinding wheel in the event of the screwbecoming seized inside the tapped hole.

In this way, the cover 11 can in all events be removed in order to gainaccess to the thermocouple column and to the sealed bearing means 16.

Machined inside the frustoconical-shaped heads of the screws 21 is anopening 21' allowing engagement of a tool for turning the screw 21during clamping which accommodates the end of a screw 18 for lifting thecompression plate 17, ensuring clamping of the thermocouple column.

Machined on the lower surface of the cover 11 are groove 32 and aprojecting rim 33 which are coaxial with the central opening in thecover 11 allowing the passage of the thermocouple column 5.

The rim 33 which projects under the cover 11 internally includes tworadially inwardly projecting parts separated by set-back partsconstituting a first set of assembly elements of a bayonet-fasteningdevice.

A clamping ring 28 is engaged via a fixing part 28a inside the groove 32and on its external lateral surface has two parts set back radiallyinwards which complement the two radially projecting parts of the rim 33which are separated by two radially projecting parts allowing a bayonetassembly between the clamping ring 28 and the cover 11.

The clamping ring 28, which has a substantially L-shaped cross-section,has an internal surface 28b which has the shape of a portion of a torusof circular section intended to come into bear against a Helicoflexgasket 30 constituting the interior sealing gasket of the thermocouplecolumn. The clamping ring 28 and the Helicoflex gasket 30 constitute theinternal part of the sealing means 16 of the thermocouple column 5. TheHelicoflex gasket 29 constitutes the external sealing gasket for thethermocouple column.

The clamping ring 28 and the Helicoflex gasket 30 are mounted on thelower surface of the cover 11 by engaging the clamping ring 28, thesurface 28b of which is in contact with the gasket 30, inside the groove32, bringing the projecting parts of the outer surface of the ring 28into coincidence with the set-back parts of the inner surface of the rim33.

The clamping ring 28 is then fixed on the cover 11 by rotating the ring28 about its axis by a quarter of a turn inside the groove 32. In thisway the clamping ring 28 is bayonet-fastened, holding the Helicoflexgasket 30 on the cover 11.

The cover 11 can then be engaged over the upper end of the thermocouplecolumn 5 in the lowered unclamped position, as represented in FIG. 4B.

The openings 27 penetrating the cover 11 are brought into coincidencewith the openings penetrating the upper part of the tubular length 8, ateach of the recesses 20. The cover is fixed on by introducing the fixingpieces 22 into the recesses 20 and screwing the screws 21 into theopenings which have been brought into coincidence belonging to the cover11, to the tubular length 8, and to the fixing pieces 22.

External sealing of the thermocouple column is provided by theHelicoflex gasket 29 clamped between the cover 11 and the upper part ofthe tubular length 8.

Internal sealing of the thermocouple column 5 is provided by compressionof the Helicoflex gasket 30 by means of the clamping piece 28 whichincludes a frustoconical lower surface 28c against which thefrustoconical surface 14 of the thermocouple column comes to bear whenthe thermocouple column is lifted by use of the compression plate 17, asrepresented in FIG. 4A.

The compression plate 17 has an annular shape, the central opening ofthe plate 17 allowing the passage of the upper part of the thermocouplecolumn 5.

The plate 17 is penetrated by three tapped openings 17' placed at 120°from one another about the axis of the plate 17, in each of whichopenings a lifting screw 18 for the compression plate 17 may be engagedand fixed.

In order to lift the thermocouple column, the compression plate 17 isengaged over the upper part of the thermocouple column 5, the two-partring 19 not being engaged in the corresponding groove of thethermocouple column.

The compression plate 17 is oriented such that the lower end parts ofthe screws 18 each engage in an opening 21' of the head of a screw 21.

The two-part ring 19 for lifting the thermocouple column is engaged inthe corresponding groove of the upper part of the thermocouple columnand comes to bear on the upper surface of the compression plate 17 whichrests on the upper surface of the cover 11, as represented in FIG. 4B.

The plate and the thermocouple column are then lifted by screwing in thescrews 18 which come to bear via their lower end part in the housings inthe frustoconical screw heads of the screws 31 for attaching the cover11.

The lifting of the plate 17 leads, via the two-part ring 19, to thelifting of the thermocouple column 5, the radially projectingfrustoconical surface 14 of which comes to bear on the lower surface 28cof the clamping ring 28, as is visible in FIG. 4A.

Subsequent movement of the plate 17 and of the thermocouple column 5causes clamping of the internal Helicoflex gasket 30, by means of thetorus-shaped internal surface 28b of the clamping ring 28, as is visiblein FIG. 4C.

The clamping ring 28 is mounted inside the groove 32 with axialclearance 34 so that this ring can move axially in order to clamp thegasket 30 which comes into sealed contact with the lower surface of thebearing cover 11 inside the groove 32, and in order to clamp the outerseal 29 against the outer surface of the thermocouple column 5. Thuscomplete sealing around the upper part of the thermocouple column 5 isensured.

The screws 18 for the lifting of the plate 17 include a cylindricalhead, by means of which they are tightened, and which is engaged insidea cup 24 including a cylindrical deformable collar integral with a baseof substantially square shape engaged and prevented from rotating andfrom moving in terms of altitude in a radial dovetail slot 25 machinedin the upper surface of the plate 17.

The heads of the screws 18 include recesses into which the collars ofthe cups 24 can be knocked over so as to prevent the screws 18 fromrotating, after the lifting of the plate 17 and the sealed clamping ofthermocouple column, as represented in FIG. 4A.

In this way clamping with the thermocouple column bearing in a sealedmanner against the cover 11 which constitutes the upper part of thebearing assembly 9 is achieved.

The sealing device for the thermocouple column perfectly withstands theconditions of pressure and of temperature which prevail inside a nuclearreactor vessel (water containing boric acid at a temperature of 310° C.under pressure of 155 bar).

It should be noted that in cases where the latitude for upwards verticalmovement of the thermocouple column is small, for example in cases wherethe thermocouple column has been deformed in use, it remains possible toprovide a perfect seal around the thermocouple column whose scope forvertical movement is reduced.

This is because the clamping ring 28 for the Helicoflex gasket 30 canadapt itself to the remaining scope for movement of the thermocouplecolumn 5.

In this way, sealed clamping of the thermocouple column 5 can beachieved in all cases, by providing a clamping ring 28 of appropriatedimensions, particularly in the axial direction.

The clamping ring can be changed very simply, by virtue of the bayonetmeans by which the clamping ring is fixed under the cover 11.

Furthermore, it should be noted that in the position for sealed clampingof the thermocouple column, the screws of the compression plate 17 andthe screws for fixing the cover 11 are all locked in a single operationby crimping the collar of the cups 24 into the recesses in the heads ofthe screws 18. The screws 18 engaged by their lower part in the openings21' in the heads of the screws 21 lock the screws 21 which cannot becomeunscrewed in operation.

After the thermocouple column has been used for a period of running ofthe nuclear reactor and after the nuclear reactor has been shut downafter its period of running, the thermocouple column and the cover 11can be removed in order to gain access to the gasket situated under thecover 11.

After the cover 11 has been removed, the clamping ring 28 may, forexample, be exchanged in order to make it suit the reduced scope foraxial movement of the thermocouple column.

The thermocouple column is removed by uncrimping the collars 24 of thescrews 18 and unscrewing the screws 18 in order to lower the compressionplate 17 as far as the position in which it is represented in FIG. 4B.

The two-part lifting ring 19 is then removed and the plate 17 is raisedso as to detach it from the upper part of the thermocouple column.

The fixing screws 21 of the cover 11 are unscrewed and thus detachedfrom the fixing pieces 22 engaged in the recesses 20 of the tubularlength 8.

In cases where a screw 21 is seized in the corresponding fixing piece22, the fixing piece 22 and the screw 21 are cut through at the externalgroove 22' of the fixing piece 22.

After the screws 21 have been unscrewed, the cover 11 is lifted andextracted via the upper part of the thermocouple column.

As can be seen in FIGS. 2, 4A and 4B, the cover 11 is penetrated bythree openings 26 arranged at 120° from one another between the openings27 which accommodate the screws 21.

Should the cover 11 remain jammed on the tubular length 8 and thethermocouple column 5 by means of the Helicoflex gaskets, after thescrews 21 have been unscrewed, the cover can be extracted by introducingand screwing lifting screws 23 into the openings 26, the lower part ofthese screws coming into contact with the upper surface of the tubularlength 8.

It is thus possible in all events to remove the cover 11.

The device according to the invention thus makes it possible to providean excellent seal of the thermocouple column irrespective of the scopefor axial movement of the thermocouple column and to gain access to thesealing gasket without removing the upper tubular length of the bearingassembly of the thermocouple column. This upper tubular length 8 cantherefore be welded permanently to the extension of the adapterpenetrating the head of the vessel or made unitary with the extension.

The arrangement of the screws of the lifting plate vertically in linewith the screws for attaching the cover with engagement of the lowerpart of the lifting screws in an opening in the head of a screw forattaching the cover makes it possible easily to lock the screws forattaching the cover, simply by preventing the screws of the compressionplate from rotating.

It is possible to use a clamping ring which has a different form fromthe one described and which includes means for fixing to the cover ofthe bearing assembly which are other than a bayonet-type fixing.

The tubular length to which the cover is fixed may have cavitiesdifferent from the three recesses which have been described, foraccommodating fixing pieces into which the screws are fixed. Forexample, a single annular cavity may be provided for accommodating thethree fixing pieces.

Finally, the sealing device according to the invention may be applied toany instrumentation column penetrating the head of a nuclear reactorvessel.

I claim:
 1. In a pressurized-water nuclear vessel having a head, aplurality of tubular adapters, each fixed into a penetration openingtraversing said head and projecting outside the head, and at least oneinstrumentation column passing through one adapter of said plurality ofadapters, a sealing device of said instrumentation column into said oneadapter, said sealing device comprising:(a) a tubular bearing assemblyfixed to an end of said adapter outside the head, in an axial extensionof said adapter, said instrumentation column being so arranged in saidbearing assembly that an end of the instrumentation column projectsoutside said bearing assembly; and (b) means for pulling on the end ofthe instrumentation column in an axial direction of the adapter in orderto bring a bearing surface of the instrumentation column into contactwith sealed bearing means arranged inside said bearing assembly; (c)said sealed bearing means comprising:(i) a clamping ring axially movablymounted inside a groove of the tubular bearing assembly and removabletherefrom, said clamping ring having a contact surface matching thebearing surface of the instrumentation column; and (ii) at least oneannular sealing gasket interposed between a bearing face of the clampingring opposite the contact surface of said clamping ring and an internalbearing surface of the tubular bearing assembly, and arranged around anexternal surface of said instrumentation column, said tubular bearingassembly including an annular cover fixed on the end of the adapterthrough which the instrumentation column passes, and comprising theinternal bearing surface of the tubular bearing assembly and said groovefor receiving a portion of the clamping ring in an assembled andremovable relationship; (iii) wherein the bearing assembly comprises atubular section fixed to the outer end of the adapter, and means forassembling the tubular section and the cover, the tubular sectionincluding at least one external peripheral cavity and at least threeopenings penetrating part of the tubular section in its axial directionand emerging in the peripheral cavity, the cover being superposed on thetubular section and including penetration openings in an extension ofthe openings in the tubular section, the means of assembling the coverand the tubular section including at least three assembly piecesinserted into the peripheral cavity of the tubular section comprisingtapped openings in the extension of the openings in the tubular sectionand in the cover, and assembly screws adapted to be introduced into thealigned openings in the tubular section and in the cover and screwedinto the assembly pieces.
 2. The sealing device according to claim 1,wherein the tubular section includes three external peripheral cavitiesarranged angularly at 120° from one another about the axis of thetubular section and three fixing pieces each engaged in an externalcavity and having a tapped opening placed in alignment with the openingspassing axially through the tubular section and the cover, for theengagement and screwing of an assembly screw.
 3. The sealing deviceaccording to claim 1, wherein each of the fixing pieces is traversed bytwo axially-directed openings, a shank which is screw-fastened to thetubular section being engaged in each of said openings, with clearance.4. The sealing device according to claim 1, wherein the means forpulling the instrumentation column consists of an annular plate engagedvia its central opening over an upper end of the thermocouple column, atwo-part ring engaged in a groove of the upper part of the thermocouplecolumn and bearing on an upper surface of the plate, and three liftingscrews engaged in axially-directed tapped openings passing through theplate and arranged at 120° from one another about the axis of the plate.